Asthma and chronic obstructive pulmonary disease (COPD) are common lung diseases characterized by chronic airway inflammation and airway obstruction. Among patient with COPD and asthma; there is a group of patients with an overlap between clinical, functional characteristics and airway inflammation patterns, named "Asthma-COPD Overlap Syndrome" (ACOS). ACOS is a syndrome characterized by reversible but persistant airflow limitation (postbronchodilator FEV1/FVC < 70%) which has some features of both asthma and COPD. ACOS should be suspected in a patient > 40 years, with smoking history, previous asthma diagnosis or history of childhood asthma who has persistant airflow limitation and reversible ariway obstruction (defined by an increase of > % 12 of FEV1 pred or increase of FEV1 > 200 mL after inhalation of 400 mcg salbutamol or 1000 mcg terbutaline). The prevalence for ACOS has been reported 11-55% in different case series to date and increases by age and is more frequent in females in different age groups. Patients with ACOS are younger than COPD patients and older than asthma patients. Frequent and severe exacerbations and related hospitalization and emergency room visits are common in ACOS and this causes an impaired quality of life. Current recommendations of guidelines for pharmacologic treatment of ACOS have been composed of a combination with optimal COPD and asthma treatment. Future therapeutic approaches should be based on endotypes. Clinical phenotype and underlying endotype driven clinical studies may be the base of ACOS guidelines.